Wrong field name to extract user name for job description structure in job_submit.lua

[rezib]

Job submit lua script expects to extract user name from username field of job_desc structure:
https://github.com/edf-hpc/slurm-llnl-misc-plugins/blob/master/job_submit/job_submit.lua#L252

It is used in several log messages and in the test for WCKEY_USER_EXCEPTION_FILE optional feature.

However, Slurm exports the username in user_name field (with an underscore) of this structure:
https://github.com/SchedMD/slurm/blob/master/src/plugins/job_submit/lua/job_submit_lua.c#L713

As a result, job_desc.username is always nil in Job submit Lua script logic and log messages.

[mchouque]

Will be fixed when commit 37cbd26 is packaged in a release.